Absolute Hydration Free Energy of Small Anions and the Aqueous pK_a of Simple Acids

Heats of formation and gas phase acidities for the simple acids and their deprotonated anions (A^- = F^-, Cl^-, Br^-, I^-, OH^-, SH^-, SeH^-, TeH^-, OCl^-, OBr^-, and OI^-) were calculated using the Feller-Peterson-Dixon (FPD) method with large basis sets including Douglass-Kroll scalar relativistic corrections. Hydration of the neutral and anionic species was predicted using the supermolecule-continuum approach, resulting in absolute hydration free energies that, when combined with calculated gas phase acidities, produce aqueous acidities and pK_a values for these simple acids that are, in general, in excellent agreement with experimental literature values. Absolute hydration free energy values converged quickly in terms of the experimental values for neutral species, requiring only four explicit H₂O molecules. HI is anomalous in that it fully dissociates ionically in a water tetramer and was treated without explicit water molecules. The hydration energies of anionic species converged more slowly and were modeled with up to 16 explicit H₂O molecules. Calculated values for ΔH_f and ΔG_gas agree with experimental values within ca. 1.2 kcal/mol, and ΔG_aq and ΔΔG_hyd agree with experimental values within ca. 2 kcal/mol in most cases.